It's the most sensitive day of the year for China's internet, the anniversary of the bloody June 4 crackdown on pro-democracy protests at Tiananmen Square, and with under two weeks to go, China's robot censors are working overtime.

Censors at Chinese internet companies say tools to detect and block content related to the 1989 crackdown have reached unprecedented levels of accuracy, aided by machine learning and voice and image recognition.

"We sometimes say that the artificial intelligence is a scalpel, and a human is a machete," said one content screening employee at Beijing Bytedance Co., who asked not to be identified because they are not authorized to speak to media.